When sin entered the world through the disobedience of man, it brought with it a curse. That curse was death. Along with death came sickness. Sickness did not exist until the fall of man. But, through Jesus' death and resurrection, the curse of sin was broken. Death was conquered. Since sickness came as a part of the curse, we can say that Jesus' sacrifice also conquered sickness and provided healing. Isaiah 53:5 tells us that by Jesus' wounds we were healed. Healing has been made possible through the atoning death of Jesus Christ.
Now, this does not mean that sickness no longer exists. We still live in a fallen world and we all experience sickness throughout our lives. But, God is a God of healing. In Exodus 15:26, God tells us that He is the Lord who heals us. When we are faced with sickness, we must always remember that it is God's nature to heal and we should pray for that healing. But, although we do have the promise of salvation through Jesus, we don't have a promise that we will be healed. God may allow us to walk through our sickness. It may be to build our faith in Him, or perhaps to build the faith of others as they see us hold on to our faith in God despite our suffering. Only God knows the reason, but we must never waver in our faith and confidence that He is able to heal.
When we find ourselves waiting patiently for healing, we must live a life of faithfulness. We can do this in a number of ways. First, we must seek communion with God through prayer. We must pray through our suffering (James 5:13). We must walk in obedience to God's Word (Deuteronomy 7:11,15).
We must wait on the Lord to give us His strength (Isaiah 40:29,31). And, we must always be encouraged in the faithfulness of our God (Psalm 89:1-2).
When healing comes, and even when it doesn't, we should always thank God for His provision of healing. We should look forward to the day when healing is no longer needed. There will be a day when all things will be made new, when everything will be perfect and sickness will be no more because the curse will no longer exist (Revelation 22:3).